Russia’s failure to let the plane land and its refusal to take responsibility for the crash are upsetting a key partner.

Azerbaijan President Ilham Aliyev said Russia is behind the Christmas Day crash of an Azerbaijan Airlines plane in Kazakhstan, which killed 38 of the 67 people aboard. The motives of the incident remain unclear.

Russian President Vladimir Putin apologized for the plane crash in Russian air space on Christmas Day that left 38 people dead in a statement that said Russia was targeting Ukrainian drones over the plane’s planned destination at the time, but stopped short of taking responsibility.